lol. ok basically there is acouple ways this will go. this is an auto mature doesnt mean sex is there i mean it could happen but i dont have it in my idea. its mature cause it'll involve death, suicide murder, and all the lovely things in the mature world

anyway first way is a demon and an angel are fighting over souls constantly and the constant encounters eventually wear them down to point were they begin discuss the war and how its affected them.
another one is a demon persuades a boy/ or girl to kill themself or commit a sin that ultimately gets the killed and when they die and he sees there soul counterpart he relizes he made a mistake in killing her and then begins to protect her and help her on her way to redemption.
another is an angel has lost his way with god and is sent back to earth to find his way back but in the process he does bad things instead but in the middle of his sinning ways he meets a girl that sets him straight and with this it doesnt sound mature but like i said with the sinning he does it'll get bad and also will be points where the demons try to persuade him to kill himself so they can have him as a demon.
but yeah these are just some of the ways
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a7x+me+girl=baby making hahaha